HAZARD ANALYSIS
3. Higher voltages are more likely to create electrical arcing While this is not a shock
trauma per se, it is related to the shock hazard since arcing may occur at the point of con- tact with the electrical conductor.
TABLE 1.1 Important Frequency Ranges of Electrical Injury
Frequency Regimen Applications Harmful effects
DC–10 kHz Low Commercial electrical power, Joule heating; destructive cell frequency soft tissue healing; trans- membrane potentials
cutaneous electrical stimulation
100 kHz– Radio Diathermy; electrocautery Joule heating; dielectric heat-
100 MHz frequency ing of proteins
100 MHz– Microwave Microwave ovens Dielectric heating of water
100 GHz
1013–1014Hz Infrared Heating; CO
2lasers Dielectric heating of water
1014–1015Hz Visible light Optical lasers Retinal injury; photochemical
reactions
1015Hz and Ionizing Radiotherapy; x-ray imaging; Generation of free radicals
HAZARDS OF ELECTRICITY 1.5
Current Magnitude. The magnitude of the current that flows through the body obeys Ohm’s law, that is,
(1.2) where I= current magnitude, amperes (A)
E= applied voltage, volts (V)
R= resistance of path through which current flows, ohms (Ω)
In Fig. 1.1 the worker has contacted a 120-V circuit when an electric drill short-circuits internally. The internal short circuit impresses 120 V across the body of the worker from the hand to the feet. This creates a current flow through the worker to the ground and back to the source. The total current flow in this case is given by the formula
(1.3) Variable R2is the resistance of the earth and for the purposes of this analysis may be
ignored. Variable R1 is the resistance of the worker’s body and includes the skin
I E R R = + 1 2 I E R = 120 Drill case R1 R2 R1= Resistance of body R2= Resistance of earth Electrical equivalent (b)
FIGURE 1.1 Electric shock current path. Electric drill
Current path
Physical circuit (a)
1.6 CHAPTER ONE
resistance, the internal body resistance, and the resistance of the shoes where they con- tact the earth.
Typical values for the various components can be found in Tables 1.2 and 1.3. Assume, for example, that a worker shown in Fig. 1.1 is wearing leather shoes and is standing in wet soil. This person is perspiring heavily and has an internal resistance of 200 Ω. From Tables 1.2 and 1.3 the total resistance can be calculated as
500Ω (drill handle) + 200 Ω (internal) + 5000 Ω (wet shoes) = 5700 Ω From this information the total current flow through the body for a 120-V circuit is calcu- lated as
(1.4) Table 1.4 lists the approximate effects that various currents will have on a 68-kilogram (kg) human being. The current flow of 21.1 mA is sufficient to cause the worker to go into
I= 120 =
5700 21 1. milliamperes(mA)
TABLE 1.3 Nominal Resistance Values for Various Materials
Material Resistance*
Rubber gloves or soles >20 MΩ
Dry concrete above grade 1–5 MΩ
Dry concrete on grade 0.2–1 MΩ
Leather sole, dry, including foot 0.1–0.5 MΩ Leather sole, damp, including foot 5–20 kΩ
Wet concrete on grade 1–5 kΩ
* Resistances shown are for 130-cm2areas.
Source: Courtesy of Ralph Lee.
TABLE 1.2 Nominal Resistance Values for Various Parts of the Human Body Resistance
Condition (area to suit) Dry Wet
Finger touch 40 kΩ–1 MΩ 4–15 kΩ
Hand holding wire 10–50 kΩ 3–6 kΩ
Finger-thumb grasp* 10–30 kΩ 2–5 kΩ
Hand holding pliers 5–10 kΩ 1–3 kΩ
Palm touch 3–8 kΩ 1–2 kΩ
Hand around 1 -inch (in) pipe (or drill handle) 1–3 kΩ 0.5–1.5 kΩ
Two hands around 1 -in pipe 0.5–1.5 kΩ 250–750Ω
Hand immersed — 200–500 Ω
Foot immersed — 100–300 Ω
Human body, internal, excluding skin — 200–1000 Ω
* Data interpolated.
Source: This table was compiled from Kouwenhoven and Milner. Permission obtained from estate of Ralph Lee.
1 2 1 2
HAZARDS OF ELECTRICITY 1.7
an “electrical hold.” This is a condition wherein the muscles are contracted and held by the passage of the electric current—the worker cannot let go. Under these circumstances, the electric shock would continue until the current was interrupted or until someone intervened and freed the worker from the contact. Unless the worker is freed quickly, tissue and mate- rial heating will cause the resistances to drop, resulting in an increase in the current. Such cases are frequently fatal.
The reader should note that the values given in this example are for illustration only. Much lower values can and do occur, and many workers have been electrocuted in exactly this same scenario.
Parts of the Body. Current flow affects the various bodily organs in different manners. For example, the heart can be caused to fibrillate with as little as 75 mA. The diaphragm and the breathing system can be paralyzed, which possibly may be fatal without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current flow are covered in later sections.
ARC
Caution: The calculation and formulas in this section are shown to illustrate the basic con- cepts involved in the calculation of arc parameters including current, voltage, and energy. The calculation of actual values for specific field conditions is a complex, safety-related procedure, and should be done only under the direction of experienced engineers.
TABLE 1.4 Nominal Human Response to Current Magnitudes
Current (60 Hz) Physiological phenomena Feeling or lethal incidence
<1 mA None Imperceptible
1 mA Perception threshold Mild sensation
1–3 mA Painful sensation
3–10 mA
10 mA Paralysis threshold of arms Cannot release hand grip; if no grip,
victim may be thrown clear (may progress to higher current and be fatal)
30 mA Respiratory paralysis Stoppage of breathing (frequently
fatal)
75 mA Fibrillation threshold 0.5% Heart action discoordinated (prob-
ably fatal)
250 mA Fibrillation threshold 99.5%
(≥5-s exposure)
4 A Heart paralysis threshold Heart stops for duration of current
(no fibrillation) passage. For short shocks, may restart on interruption of current (usually not fatal from heart dysfunction)
≥5 A Tissue burning Not fatal unless vital organs are
burned
Notes: (1) This data is approximate and based on a 68-kg (150-lb) person. (2) Information for higher current lev-
els is obtained from data derived from accident victims. (3) Responses are nominal and will vary widely by individual. Source: Courtesy of Ralph Lee.
1.8 CHAPTER ONE
Definition and Description
ANSI/IEEE Std 100-1988 defines arc as: “A discharge of electricity through a gas, nor- mally characterized by a voltage drop in the immediate vicinity of the cathode approxi- mately equal to the ionization potential of the gas.”2
A similar definition, perhaps more useful in the discussion of electrical safety is given in the glossary of this handbook as: “The heat and light energy release that is caused by the electrical breakdown of and subsequent electrical discharge through an electrical insulator such as air.”
Electric arcing occurs when a substantial amount of electric current flows through what pre- viously had been air. Since air is a poor conductor, most of the current flow is actually occur- ring through the vapor of the arc terminal material and the ionized particles of air. This mixture of super-heated, ionized materials, through which the arc current flows, is called a plasma.
Arcs can be initiated in several ways:
● When the voltage between two points exceeds the dielectric strength of the air. This can happen when overvoltages due to lightning strikes or switching surges occur.
● When the air becomes superheated with the passage of current through some conductor.
For example, if a very fine wire is subjected to excessive current, the wire will melt, superheating the air and causing an arc to start.
● When two contacts part while carrying a very high current. In this case, the last point of contact is superheated and an arc is created because of the inductive flywheel effect.
Electric arcs are extremely hot. Temperatures at the terminal points of the arcs can reach as high as 50,000 kelvin (K). Temperatures away from the terminal points are somewhat cooler but can still reach 20,000 K. Although the specific results of such temperatures will vary depending on things such as distance from the arc, ambient environmental conditions, and arc energy; anecdotal evidence supported by experimental results developed by the Institute of Electrical and Electronics Engineers (IEEE) clearly shows the following:
● The heat energy of an electrical arc can kill and injure personnel at surprisingly large distances. For example, second-degree burns have been caused on exposed skin at dis- tances of up to 12 feet (ft) or (3.6 meters [m]) and more.
● Virtually all types of clothing fibers can be ignited by the temperatures of electrical arcs.
Clothing made of non-flame resistant fibers will continue to burn after the arc source has been removed and will continue to cause serious physical trauma. Table 1.5 shows the ignition temperature of various fabrics and identifies those that will support combustion after the arc energy is gone.
TABLE 1.5 Ignition Temperatures and Characteristics of Clothing Fibers
Melt Decomposition Ignition Burning
Fiber temperature temperature temperature temperatures
Cotton/Rayon* N/A 554/581 752/788 1562
Polyester 482 734 1040 1337
Wool N/A 446 1112 Unknown
Nylon 6,6 490 653 990 1607
PBI N/A 860 N/A N/A
NOMEX N/A 900 N/A N/A
* FR treatment of cotton of rayon does not affect ignition temperatures.
All temperatures are expressed in °F. Please note that polyester ignites at a higher temperature and burns at a lower temperature than cotton. This shows the fallacy of using untreated cotton as an FR garment.
HAZARDS OF ELECTRICITY 1.9 The amount of energy, and therefore heat, in an arc is proportional to the maximum avail- able short circuit volt-amperes in the system at the point of the arc. Calculations by Ralph Lee indicate that maximum arc energy is equal to one-half the available fault volt-amperes at any given point.3
Later research by Neal, Bingham, and Doughty show that while the max- imum may be 50 percent, the actual value will usually be somewhat different depending on the degree of distortion of the waveform, the available system voltage, and the actual arc power factor.4
The same research also shows that enclosing the arc to create a so-called “arc in the box” focuses the incident arc energy and increases its effect by as much as threefold.4,5
The arc energy determines the amount of radiated energy and, therefore, the degree of injury. The arc energy will be determined by the arc voltage drop and the arcing current. After the arc is established, the arc voltage tends to be a function of arc length; conse- quently, the arc energy is less dependent on the system voltage and more dependent on the magnitude of the fault current. This means that even low voltage systems have significant arc hazard and appropriate precautions must be taken. Figures 1.2 and 1.3 show the results of two experiments that were conducted with manikins exposed to electric arcs. As can be seen, both high and low voltages can create significant burns.
Arc Energy Release
Arc energy is released in at least three forms—light, heat, and mechanical. Table 1.6 describes the nature of these energy releases and the injuries that they cause. Note that light and heat tend to cause similar injuries and will, therefore, be treated as one injury source in later calculations. Also note that mechanical injuries are usually categorized as blast injuries, even though the ultimate cause is the electric arc.
To be conservative in arc energy release calculations, two assumptions must be made:
1. All arc energy is released in the form of heat measured in cal/cm2
or J/cm2
. The reader should remember that this assumption is made solely for the purpose of analyzing electric arc thermal injury. Other hazards such as shock and blast are considered separately.